Patent Document 1 discloses a waste-gate valve device with a waste-gate channel bypassing a turbine formed into a tapered shape widened from an inlet toward an outlet, with a valve body (waste-gate valve) for closing the outlet of the waste-gate channel including a protrusion of such a height that the protrusion is flush with an inlet-side wall surface of the waste-gate channel when the waste-gate channel is closed. With the above waste-gate valve device, when the valve body closes the waste-gate channel, the upper surface of the protrusion is flush with the inlet-side wall surface of the waste-gate channel, which makes it possible to reduce turbulence in an exhaust-gas flow at an inlet side of the turbine. Further, with the waste-gate channel having a tapered shape widened from the inlet toward the outlet, a larger gap may be formed between a side surface of the protrusion and an inner peripheral surface of the waste-gate channel even when the valve body is slightly open. Thus, it is possible to achieve a larger opening area for the waste-gate channel.
Patent Document 2 discloses a waste-gate valve device including a tapered portion at an outlet side of a waste-gate channel bypassing a turbine, so that a flow-path cross-sectional area gradually increases. With the above waste-gate valve device, a pressure-reduction region is formed between the tapered portion and a valve body (waste-gate valve), and the speed of an exhaust-gas flow bypassing the turbine increases in the pressure-reduction region, which makes it possible to reduce a force, especially a moment, applied to the valve body.
Patent Document 3 discloses a waste-gate valve device with a valve body (waste-gate valve) for closing an outlet of a waste-gate channel, the valve body including a protrusion on a surface adjacent to the waste-gate channel (a seating surface) formed to have a cross section of a “+” shape. With the above waste-gate valve device, it is possible to considerably reduce a negative pressure generated in clearance between the valve body and a valve seat due to turbulence of an exhaust flow flowing through a slight gap (clearance) between the valve body and the valve seat (outlet) at the outlet of the waste-gate channel.